Add support for kitting/assembly

Allow Delivrd users to define kits/assemblies. 

The user will define a product which is a kit or assembly, and then 'child' products to that kit.

A kit can be composed of several pieces of the same product, or one or more pieces of several products.

The end customer orders the kit/assembly product, but in picking process, picker has to pick the 'child' products, or components.

Inventory of components is reduced, while the kit/assembly product is issued from sales order.

Also, user should be able to know how many kits/assemblies he can assemble based on inventory quantities of components.